Syria's army has rushed extra troops to Idlib after a strategic town in the northwestern province was seized by rebels, cutting off routes to the embattled commercial capital Aleppo, a watchdog said Wednesday.

"Helicopters are overflying the area," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, without elaborating on how many additional troops were on their way to the town of Maaret al-Numan.

The Britain-based Observatory said six civilians were killed in army shelling of Maaret al-Numan on Tuesday, while 12 rebel fighters died in the fight for control of the town.

The Observatory had on Tuesday reported that rebels overran Maaret al-Numan, on the highway linking Damascus with Aleppo, after a fierce 48-hour gunbattle and heavy shelling.

"Regular forces pulled back from all of their checkpoints around Maaret al-Numan, except for one at the entrance of the town," said Abdel Rahman.

"This is a strategic location on the route from Damascus to Aleppo. All the regime reinforcements headed to Aleppo must pass through Maaret al-Numan."